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Quad 405 transistors using more power
Quad 405 transistors using more power
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Old 7th March 2018, 05:07 AM   #1
goodguys is offline goodguys  United Kingdom
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Default Quad 405 transistors using more power

I swapped out the stock toshiba 2sd424 transistors in a quad 405 amplifier for more modern motorolla MJ15003,
also replaced the stock RCA 40872 driver transistors with the Motorola MJE15031
Intrestingly, I also swapped out all the resistors with 1% metal film, replaced the bridge rectifier, capacitors with panasonics, used heavier wiring,
but the one mod that gave me the biggest increase in sound quality was changing the stock 350va transformer for a 700va.
It appears strange that this, the most important mod, is not mentioned on any of the websites such as dada, diy forum etc, and i am wondering if it just me, where all the power is going to? why the need for such a large transfomer? Have i done something wrong.
Could it be that the motorolla transistors are all a lot more power hungry than the stock ones and are demanding more power, did i make a mistake by swapping out the transistors. Could you comment.

Thanks.
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Old 7th March 2018, 07:14 AM   #2
goodguys is offline goodguys  United Kingdom
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Would that explain the improvment with using a larger transformer, the transistors are the only thing i can see which would pull so much current
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Old 7th March 2018, 07:23 AM   #3
jan.didden is online now jan.didden  Europe
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Quote:
Originally Posted by goodguys View Post
Would that explain the improvment with using a larger transformer, the transistors are the only thing i can see which would pull so much current
I thought I just explained that transistors can't just 'pull current'. The driver stages open up the output devices for whatever the speaker voltage has to be, so in itself heavier transistors cannot draw more current unless driven heavier giving louder output.

It is tricky to draw conclusions from such personalized casual listening, especially with several changes at the same time. So it might not be the transformer that makes the change.

At any rate, the xformer could make a difference if there would be a supply voltage drop with loud music with the smaller xformer. Only could, because there are other factors to consider. It's not a straightforward thing to answer conclusively.

Jan
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Last edited by jan.didden; 7th March 2018 at 07:26 AM.
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Old 7th March 2018, 02:01 PM   #4
Mooly is offline Mooly  United Kingdom
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Quad 405 transistors using more power
You may find this thread interesting:
Quad 405-2 Overheating on right channel.
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Old 8th March 2018, 12:03 AM   #5
goodguys is offline goodguys  United Kingdom
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Hi. Thanks for the replies. I just spoke to Cantebury Windings over here in britain who are transformer specalists and they were saying a 350va transformer is too small for a quad 405 and recommended a 700va, i argued with them but they were adamant, saying that they had worked on quads before. Quads were built to a budget and the large transformer would have taken too much money.
This kind of corroborates my own findings, the quad just sounds so much better with the bigger transformer, it is an obvious improvement.
Bernd ludwig in his papers says changing the stock toshiba transistors for more modern faster ones is pointless as the stock ones are fast enough as it is, he noticed no improvement in sound quality, but there are a few forum posts from hobbyists saying they noticed big improvements. Do you have an opionion. Sorry, i am getting so much conflicting information.
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Old 8th March 2018, 02:29 AM   #6
Ian Finch is offline Ian Finch  Australia
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You are right, Canterbury Windings must have their wires crossed or they are overstocked with large cores . A 700 VA transformer is much larger than specified even for the larger 606, 909 models. I don't accept the reasoning for such a monster that probably wouldn't fit the case anyway. Quad 405-2 Transformer Substitute
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Old 8th March 2018, 06:45 AM   #7
goodguys is offline goodguys  United Kingdom
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Thanks, much appreciated.
One last thing, what do you think about removing the two 10kuf capacitors and replacing with 6 x 1800uf.
The 10kuf cap has an esr of 0.03 ohms, and paralleling the six caps will give me 0.004 ohms.
There is a lot of information about paralleling caps to lower esr and improve performance, but in real life it looks like replacing the 10kuf cap will lower esr by 0.026 of an ohm. Will this fraction of an ohm do anything to improve performance.
Finally at the point where the positive and negative cables from the caps meet the amp board, there are two 0.1uf film caps, would placing a 2200uf cap across each film cap benefit sound quality. It would provide a local power supply and prevent any interference between the two power amp boards, but in real life would this be worth the hassle and cost. Would it be audible.
Thanks
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Old 8th March 2018, 08:47 AM   #8
jan.didden is online now jan.didden  Europe
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Quote:
Originally Posted by goodguys View Post
Hi. Thanks for the replies. I just spoke to Cantebury Windings over here in britain who are transformer specalists and they were saying a 350va transformer is too small for a quad 405 and recommended a 700va, i argued with them but they were adamant, saying that they had worked on quads before. Quads were built to a budget and the large transformer would have taken too much money.
This kind of corroborates my own findings, the quad just sounds so much better with the bigger transformer, it is an obvious improvement.
Bernd ludwig in his papers says changing the stock toshiba transistors for more modern faster ones is pointless as the stock ones are fast enough as it is, he noticed no improvement in sound quality, but there are a few forum posts from hobbyists saying they noticed big improvements. Do you have an opionion. Sorry, i am getting so much conflicting information.
There were also some posts that described oscillations and instability with faster output devices. That at least makes engineering sense and you should not do that unless you have a fully equipped lab and are prepared to replace lots of smoked parts.

Jan
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Old 8th March 2018, 09:53 PM   #9
405man is offline 405man  Scotland
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Has anyone measured the VA rating of the 405 transformer,. I tried loading one until the voltage dropped by 5% but at 500W the load resistor was smoking and I had to abandon the test before I set off the fire alarm.

Stuart
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Old 8th March 2018, 10:16 PM   #10
TonyTecson is offline TonyTecson  Philippines
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Quad 405 transistors using more power
Quote:
Originally Posted by goodguys View Post
Would that explain the improvment with using a larger transformer, the transistors are the only thing i can see which would pull so much current
it is the speaker loads and the demand of the input signals that dictate how much power is drawn...
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